1. Field of the Invention
This invention generally relates to electrochemical batteries and, more particularly, to a carbonaceous anode for use with sodium-ion and potassium-ion batteries.
2. Description of the Related Art
Although sodium (Na) metal is a good choice for sodium-ion batteries (NIBs), its application in commercial batteries is constrained by safety issues such as flammability, dendrite growth during charge/discharge, and a low melting point. As an alternative to metallic Na, carbonaceous anodes have emerged as attractive candidates for NIBs.
In general, carbonaceous materials have three allotropes, which are diamond, graphite, and buckminsterfullerene [1]. In their application to lithium-ion batteries (LIBs), graphite and its disordered forms are both popular and practical anode materials. Graphite has a typical layered structure into/from which lithium ions (Li+) can reversibly intercalate/deintercalate. Due to the larger sizes of sodium ions (Na+) and potassium ions (K+) relative to lithium ions (Li+), graphite with a small interlayer distance is not appropriate for sodium/potassium intercalation and, consequently, demonstrates a low capacity [2]. Under certain experimental conditions, amorphous carbonaceous materials can be prepared. Depending on the degree of crystallinity, these materials can be further classified as either “soft carbon” (SC, graphitizable carbon) or “hard carbon” (HC, non-graphitizable carbon). Indeed, amorphous carbonaceous materials have demonstrated good performance as anodes in NIBs. Carbon black, a type of soft carbon, was reported as the anode material in NIBs for which sodium was shown to be reversibly inserted into its amorphous and non-porous structures [3], while its reversible capacity was ˜200 milliampere hours per gram (mAh/g) between 0 V-2 V (vs. Na/Na+). Since the carbon black has almost negligible porosity, it is believed that its large external surface area facilitates the reaction with sodium. However, the large surface area is also detrimental in terms of a large irreversible capacity for the carbon black anode.
To overcome the small capacities and low coulombic efficiencies of soft carbon materials, hard carbons are being intensively investigated as a NIB anode and have demonstrated reversible capacities exceeding 250 mAh/g [4, 5]. Sodiation of a hard carbon electrode includes two distinct processes. At the high voltage range (slope region), Na+ inserts into the parallel graphene layers. At the low voltage range (plateau region), Na+ intercalates into the pores of hard carbon. However, noteworthy is the fact that the low voltage plateau is very close to 0 V vs. Na/Na+ so that sodium electroplating on hard carbon electrode can proceed when high currents for HC sodiation are applied. At the same time, the high current leads to a reduced capacity due to high polarization for the hard carbon electrode. As a result, conductive carbon black was added into the HC electrode in order to reduce the electrode resistance [6, 7]. Of course, carbon black can contribute to a large irreversible capacity (low coulombic efficiency, CE) for the hard carbon electrode at the first cycle.
A comparison by the Applicants on the impact of several types of conductive carbon additives on hard carbon electrode performance has unambiguously determined that CE is directly related to the surface area of the carbon additives. More specifically, high surface areas for conductive carbon additives were correlated with appreciable irreversible capacities due to solid electrolyte interface (SEI) layer formation on the electrode. In addition, the diverse functional groups on the surfaces of these carbon additives can also contribute to both the high irreversible capacities and low corresponding CE for hard carbon electrode.
It would be advantageous if a hard carbon electrode could be prepared for use in sodium-ion or potassium-ion batteries that demonstrated a large capacity at high currents, as well as high CE at the first cycle.    [1] Z. Ogumi and M. Inaba, “Electrochemical Lithium Intercalation within Carbonaceous Materials: Intercalation Processes, Surface Film Formation, and Lithium Diffusion”, Bulletin of the Chemical Society of Japan, 71(1998) 521-534.    [2] M. M. Doeff, Y. Ma, S. J. Visco, and L. C. De Jonghe, “Electrochemical Insertion of Sodium into Carbon”, Journal of the Electrochemical Society, 140(1993), L169-L170.    [3] R. Alcántara, J, M. Jiménez-Mateos, P. Lavela, and J. Tirado, “Carbon Black: a Promising Electrode Material for Sodium-Ion Batteries”, Electrochemistry Communications, 3 (2001), 639-642.    [4] X. Xia and J. R. Dahn, “Study of the Reactivity of Na/Hard Carbon with Different Solvents and Electrolytes”, Journal of the Electrochemical Society, 159 (2012), A515-A519.    [5] S. Kuze, J.-i. Kageura, S. Matsumoto, T. Nakayama, M. Makidera, M. Saka, T. Yamaguchi, T. Yamamoto, and K. Nakane, “Development of a Sodium Ion Secondary Battery, SUMITOMO KAGAKU, 2013, 1-13.    [6] A. Ponrouch, A. R. Goni, and M. R. Palacín, “High Capacity Hard Carbon Anodes for Sodium Ion Batteries in Additive Free Electrolyte”, Electrochemistry Communications, 27(2013), 85-88.    [7] S. Komaba, W. Murata, T. Ishikawa, N. Yabuuchi, T. Ozeki, T. Nakayama, A. Ogata, K. Gotoh, and K. Fujiwara, “Electrochemical Na Insertion and Solid Electrolyte Interphase for Hard-Carbon Electrodes and Application to Na-Ion Batteries”, Advanced Functional Materials, 21(2011), 3859-3867.